The present invention generally relates to silicon-on-insulator semiconductor device structures and methods and, more particularly, to a simplified undercut etching method based on the use of doped reach-through regions to isolate such device structures from each other on the same chip.
Undercut etching methods to remove selectively a buried doped semiconductive region to form submerged spaces for the insertion of insulating material are well-known in the silicon-on-insulator (SOI) art. The various methods differ in complexity particularly as to how access is gained to reach and remove the buried region and as to how the overlying region (to be isolated) is supported during the process of removing the buried region.
For example, U.S. Pat. No. 4,888,300, issued to Gregory N. Burton on Dec. 19, 1989 for Submerged Wall Isolation of Silicon Islands, teaches the use of trenches in an epitaxial layer to provide access to a differently doped buried layer. While supporting the portion of the epitaxial layer adjacent the trench (by an oxide bridge), the underlying region of the buried layer is etched away to form a cavity under the active area. Five etching steps are disclosed to accomplish the accessing, supporting and removal steps for forming the cavity and for completely isolating the silicon islands from the substrate. Copending patent application, Ser. No. 07/781,429, filed Oct. 23, 1991 in the names of K. D. Beyer et al. for Buried Air Dielectric Isolation of Silicon Islands and assigned to the present assignee also utilizes trench and oxide bridging steps to provide for the accessing and removal of the buried region material while supporting the active area. Three etching steps are disclosed for forming completely isolated silicon islands.